Method and system for compensating for chromatic dispersion in an optical network

ABSTRACT

The present invention provides a method and system for compensating for chromatic dispersion in an optical network. The method and system includes analyzing an optical signal at a first location in the optical network and determining if an eye of the optical signal has a desired shape at the first location. The method and system further includes automatically adjusting a dispersion of the optical signal at a second or at any location in the optical network if the optical signal does not have the desired shape; providing feedback concerning the shape of the adjusted optical signal at the first location; and automatically readjusting the dispersion of the optical signal at the second location based upon the feedback until the optical signal has the desired shape at the first location. The method and system of the present invention automatically compensates for dispersion in an optical system. This saves the network operator considerable time and raises the reliability. The method and system could be used to provide a continuous range of dispersion compensation to particular locations in the network or to the network as a whole, and to store information in a database to be used to produce dispersion maps for the network. It also provides the ability to dial in dispersion for changing fiber characteristics and pre-tuning in a switching of wavelengths from one system to another. Finally, it avoids extraordinary expense by allowing the approximation of the installed fiber dispersion without requiring field measurements.

FIELD OF THE INVENTION

[0001] The present invention relates to fiber optic networks, and moreparticularly to chromatic dispersion compensation in fiber opticnetworks.

BACKGROUND OF THE INVENTION

[0002] Fiber optic networks are becoming increasingly popular for datatransmission due to their high speed, high capacity capabilities. FIG. 1illustrates a simplified optical network 100. A fiber optic network 100could comprise a main loop 150, which connects primary locations, suchas San Francisco and New York. In between the primary locations arelocal loops 110, 120, which connect with the main loop 150 at connectorpoints 140 and 160. A local loop could be, for example, an opticalsystem servicing a particular area. Thus, if local loop 110 isSacramento, an optical signal would travel from San Francisco, add anddrop channels with Sacramento's signal at connector point 140, and thenew signal would travel forward to connector point 160 where channelsare added and dropped with local loop 120, and eventually to New York.Within loop 110, optical signals would be transmitted to variouslocations within its loop, servicing the Sacramento area. Localreceivers 170 would reside at various points within the loop to convertthe optical signals into signals in the appropriate protocol format.Loops 110 and 120 may also exchange channels directly with each otherthrough a connector point 130 between them.

[0003] A common and well-known problem in the transmission of opticalsignals is chromatic dispersion of the optical signal. Chromaticdispersion refers to the effect where the channels within a signaltravel through an optic fiber at different speeds, i.e.; longerwavelengths travel faster than shorter wavelengths. This is a particularproblem becomes more acute for data transmission speeds higher than 2.5gigabits per second. The resulting pulses of the signal will bestretched, possibly overlap, and make it more difficult for a receiverto distinguish where one pulse begins and another ends. This seriouslycompromises the integrity of the signal.

[0004] A conventional solution to this problem is the use of fixeddispersion compensators at various locations in the network as needed.These devices compensate for a fixed dispersion value by canceling thedispersion in the fiber link. The difficulty with using fixed dispersioncompensators is that an optical link or network is rarely uniform.Different systems in the network may use different types of fiber, aswell as different types of receivers with different tolerances. Thefibers within a system may be of different lengths necessitated bylandscapes, building locations, etc. Also, different systems may containdevices from different vendors, each with its own dispersion tolerance.Thus, in order to obtain as close to optimum dispersion compensationthrough the entire system, the dispersion must be manually determinedfor every fiber and optical in the system, and a dispersion compensatorwith the appropriate fixed value must be purchased and installed. Thissolution is costly to the network operator in both money and time. Manyhours of human labor must be expended to measure the dispersion of eachfiber in the system and to order, inventory, install, and setup thefixed dispersion compensators. To do the job properly at extremely highbit rates, a network operator must remove transmission traffic from thefiber link, measure the dispersion in the fiber link, and then manuallyinsert the fixed dispersion compensator. Many operators “guess” at thedispersion based upon the length of the fiber and statistics ofdispersion. They then order a fixed dispersion compensator, whichapproximates the dispersion. For example, assume the residual dispersionusing conventional dispersion compensation at the end of a fibertransmitting a standard NRZ pulse format has a value of 1200 ps/nm. The“guess” method will work for a pulse of 2.4 Gb/s, will be difficult toachieve at 10 Gb/s, and will not work at 40 Gb/s. Thus, the operatormust “guess” within the dispersion tolerance. In general, if theresidual dispersion of the transmission link is less than the dispersiontolerance of the Transceiver, the system will operate properly. Atextremely high bit rates such as 40 Gb/s, meeting this condition will beextremely rare.

[0005] Accordingly, there exists a need for a method and system forautomatically compensating for chromatic dispersion in an opticalnetwork, which does not require the manual determination, installationand purchase of a dispersion compensation of a fixed value. The methodwill save network operators both money and time. The present inventionaddresses such a need.

SUMMARY OF THE INVENTION

[0006] The present invention provides a method and system forcompensating for chromatic dispersion in an optical network. The methodand system includes analyzing an optical signal at a first location inthe optical network and determining if an eye of the optical signal hasa desired shape at the first location. The method and system furtherincludes automatically adjusting a dispersion of the optical signal at asecond or at any location in the optical network if the optical signaldoes not have the desired shape; providing feedback concerning the shapeof the adjusted optical signal at the first location; and automaticallyreadjusting the dispersion of the optical signal at the second locationbased upon the feedback until the optical signal has the desired shapeat the first location. The method and system of the present inventionautomatically compensates for dispersion in an optical system. Thissaves the network operator considerable time and raises the reliability.The method and system could be used to provide a continuous range ofdispersion compensation to particular locations in the network or to thenetwork as a whole, and to store information in a database to be used toproduce dispersion, maps for the network. It also provides the abilityto dial in dispersion for changing fiber characteristics and pre-tuningin a switching of wavelengths from one system to another. Finally, itavoids extraordinary expense by allowing the approximation of theinstalled fiber dispersion without requiring field measurements.

BRIEF DESCRIPTION OF THE FIGURES

[0007]FIG. 1 illustrates a conventional optical network.

[0008]FIG. 2 illustrates a preferred embodiment of a system using themethod of providing tunable dispersion compensation in accordance withthe present invention.

[0009] FIGS. 3A-3C illustrates possible locations of a tunabledispersion compensator in accordance with the present invention.

[0010]FIG. 4 illustrates a conventional eye diagram of an opticalsignal.

[0011]FIG. 5 is a flow chart illustrating a preferred embodiment of themethod of providing tunable dispersion compensation in accordance withthe present invention.

[0012]FIG. 6 is a flow chart illustrating in more detail the preferredembodiment of the method of providing tunable dispersion compensation inaccordance with the present invention.

[0013]FIG. 7 illustrates a first preferred embodiment of a tunabledispersion compensator in accordance with the present invention.

[0014]FIG. 8 illustrates in more detail the operation of the firstpreferred embodiment of a tunable dispersion compensator in accordancewith the present invention.

[0015]FIG. 9 illustrates a second preferred embodiment of a tunabledispersion compensator in accordance with the present invention.

[0016]FIG. 10 illustrates a third preferred embodiment of a tunabledispersion compensator in accordance with the present invention.

[0017]FIG. 11 is a flow chart illustrating a method of providingdispersion compensation to an optical network in accordance with thepresent invention.

[0018]FIG. 12 is a flow chart illustrating a first preferred embodimentof the method of providing dispersion compensation to an optical networkin accordance with the present invention.

[0019]FIG. 13 is a flow chart illustrating a second preferred embodimentof the method of providing dispersion compensation to an optical networkin accordance with the present invention.

[0020]FIG. 14 is a flow chart illustrating a third preferred embodimentof the method of providing dispersion compensation to an optical networkin accordance with the present invention.

DETAILED DESCRIPTION

[0021] The present invention is a method for compensating for chromaticdispersion in an optical network. The following description is presentedto enable one of ordinary skill in the art to make and use the inventionand is provided in the context of a patent application and itsrequirements. Various modifications to the preferred embodiment will bereadily apparent to those skilled in the art and the generic principlesherein may be applied to other embodiments. Thus, the present inventionis not intended to be limited to the embodiment shown but is to beaccorded the widest scope consistent with the principles and featuresdescribed herein.

[0022] To more particularly describe the features of the presentinvention, please refer to FIGS. 2 through 14 in conjunction with thediscussion below.

[0023] The method of the present invention uses a tunable dispersioncompensator (TDC) in an optical network. FIG. 2 illustrates a preferredembodiment of a system 200, which uses the method of the presentinvention. The network 200 uses amplifiers 202 to compensate for signalloss or attenuation, a commonly known problem in optical networks. Ateach location of the amplifiers 202, TDC's 204, 206 are used.

[0024] TDC's may also be located at other places in the network. FIGS.3A-3C illustrate other possible locations. For example, as illustratedin FIG. 3A, TDC's 302 and 304 may be located at the transmitting end ofthe amplifiers 306 and 308 respectively. As illustrated in FIG. 3B,TDC's 310 and 312 may be located at the receiving end of the amplifiers314 and 316 respectively. As illustrated in FIG. 3C, TDC 318 may belocated in-between amplifiers 320 and 322. Although the variations ofTDC locations are as illustrated in FIGS. 2-3C, one of ordinary skill inthe art will understand that TDC's may be located elsewhere in thenetwork without departing from the spirit and scope of the presentinvention.

[0025] The TDC's 204, 206 of FIG. 2 will be used in the followingdiscussion of the features of a TDC in accordance with the presentinvention. However, this discussion is applicable to any of thedisclosed TDC's 302, 304, 310, 312, 318.

[0026] Returning to FIG. 2, the TDC's 204, 206 are automatically tunedby a performance monitoring mechanism 210 to fine tune the shape of theoptical signal until an optimum shape is obtained. In the preferredembodiment, an Eye Diagram Analyzer 210 (EDA) at the receiver 212analyzes the optical signal at the receiver 212 and feeds theinformation to the TDC's 204, 206 through a communications link 208. TheEDA 210 measures the area of the “eye” 402 of the optical signal 404, asillustrated in FIG. 4. The “eye” 402 is the area between the peaks andopposing of the optical signal 404. For optimum performance, an optimumeye area should be obtained. An optimum area is the maximum openingbetween the NRZ pulse so that the receiver 212 has the most ease inpicking the voltage and time coordinates. The eye diagram should, byanalogy, look wide open similar to the human eye when fully open. TheNRZ pulse and its eye area are well known in the art and will not befurther described here. When chromatic dispersion distorts the signal,the eye 402 does not have an optimum area. If the area is too small oris skewed or distorted, the receiver 212 will not be able to determineif the signal is carrying a “1” or a “0” and may have difficultylocating the eye center, compromising the reliability of the data on thesignal. This problem is well known in the art. The use of an EDA tomeasure the eye area is also well known in the art. An example is theEDA manufactured by HEWLETT-PACKARD.

[0027] Although the present invention is described with the EDA locatedat the receiver, one of ordinary skill in the art will understand thatthe EDA may be located elsewhere in the network without departing fromthe spirit and scope of the present invention. For example, an EDA maybe located at each amplifier in the network such that the optical signalbetween the EDA and a prior TDC may be analyzed. For another example,instead of installing an EDA into the network, the EDA may be carried bya technician to the physical location of the amplifier, manually connectthe EDA to the network at that location, and then optimize the opticalsignal. Other manners of utilizing the EDA are possible.

[0028] Although the present invention is described as using the EDA asthe method of analyzing the shape of the optical signal, one of ordinaryskill in the art will understand that other methods of analyzing theshape of the optical signal may be used without departing from thespirit and scope of the present invention. Examples of other methodsinclude the Optical Signal to Noise measurement, the dBQ measurement, orthe Bit Error Rate measurement. These methods are well known in the artand will not be described here.

[0029]FIG. 5 is a simple flowchart illustrating the preferred embodimentof the method for tunable dispersion compensation in accordance with thepresent invention. First, an optical signal at a first location (such asat the receiver) in the optical network is analyzed before any tuninghas begun, via step 502. Next, it is determined if the eye of theoptical signal has a desired shape, via step 504. Shape determination iswell known to the skilled at art. Then, the dispersion of the opticalsignal at a second location upstream from the first location in theoptical network (such as at the tunable dispersion compensator) isautomatically adjusted if the eye does not have a desired shape, viastep 506. Feedback is then provided concerning the eye shape of theadjusted optical signal at the second location, via step 508.Thereafter, the dispersion of the optical signal is readjusted at thesecond location based upon the feedback until the eye at the firstlocation has the desired shape, via step 510.

[0030]FIG. 6 is a flowchart illustrating in more detail the preferredembodiment of the method of the present invention. For illustrativepurposes only, assume that the EDA 210 (FIG. 2) is located at a receiver212 and the TDC 204 is located at amplifier 202. First, the EDA 210obtains a set of parameters for the system, via step 610. In thepreferred embodiment, the parameters obtained by the EDA 210 include thefiber type for the system, via step 612; the fiber link distance, i.e.,the distance between the TDC 204 and the receiver 212, via step 614; andthe dispersion tolerance of the receiver 212, via step 616. Using theparameters, the EDA 210 then calculates an initial set point forcompensating the dispersion in the optical signal as it enters thesystem, via step 620. The EDA 210 sends a command to the TDC 204 throughthe communications link 208 to adjust the dispersion compensation to theset point, via step 530. Once adjusted, the EDA 210 analyzes theadjusted optical signal, via step 640. The EDA 204 then decides if theeye has the desired shape, via step 650. If not, then the EDA 210calculates a new set point and commands the TDC 204 to adjust itsdispersion compensation by a certain amount to reach the new set point,via step 660. The signal then is again analyzed by the EDA 210, via step640. This analysis and adjustment continues until the desired eye shapeis obtained.

[0031]FIGS. 7 and 8 illustrate a first preferred embodiment of a TDC,which may be used, with the method of the present invention. The firstembodiment of the TDC has been disclosed in co-pending U.S. patentapplication entitled, “VIRTUALLY IMAGED PHASED ARRAY (VIPA) HAVING AVARYING REFLECTIVITY SURFACE TO IMPROVE BEAM PROFILE”, Ser. No.09/114,071, filed on Jul. 13, 1998, assigned to the assignee of thepresent application and is incorporated by reference herein.

[0032] The TDC 700 in FIG. 7 includes a virtually imaged phased array(VIPA) 240 in combination with a reflecting device 254, such as amirror, to produce chromatic dispersion. The VIPA 240 has a firstsurface 242 with a reflectivity of, for example, approximately 100%, anda second surface 244 with a reflectivity of, for example, approximately98%. The VIPA 240 also includes a radiation window 247. The VIPA 240receives an input light having a respective wavelength within acontinuous range of wavelengths. The VIPA 240 causes multiplereflections of the input light to produce differential delay and therebyform an output light signal. The output light is spatiallydistinguishable from an output light formed for an input light havingany other wavelength within the continuous range of wavelengths.

[0033] As illustrated in FIG. 7, a light is output from a fiber 246,collimated by a collimating lens 248 and line-focused into VIPA 240through radiation window 247 by a cylindrical lens 250. The line intowhich the light is focused is referred to in this specification as the“focal line”. The VIPA 240 then produces a collimated light 251, whichis focused by a focusing lens 252 onto a mirror 254. Mirror 254 can be amirror portion 256 formed on a substrate 258.

[0034] Mirror 254 reflects the light back through focusing lens 252 intothe VIPA 240. The light then undergoes multiple reflections in VIPA 240and is output from radiation window 247. The light output from radiationwindow 247, travels through cylindrical lens 250, and collimating lens248 and is received by fiber 246.

[0035] Therefore, light is output from VIPA 240 and reflected by mirror254 back into VIPA 240. The light reflected by mirror 254 travelsthrough the path, which is exactly opposite in direction to the paththrough which it originally traveled. Different wavelength components inthe light are focused onto different positions on mirror 254, and arereflected back to VIPA 240. As a result, different wavelength componentstravel different distances, to thereby produced adjustable chromaticdispersion. The VIPA 240 can be used to compensate for chromaticdispersion in all channels of a wavelength division multiplexed light.

[0036]FIG. 8 illustrates in more detail the operation of the VIPA 240 inFIG. 7. Assume a light having various wavelength components is receivedby the VIPA 240. The VIPA 240 will cause the formation of virtual images260 of beam waist 262, where each virtual image 260 emits light. A “beamwaist”, as used in this specification, is the width of the focal line, across-section of which is displayed in FIG. 8.

[0037] As illustrated in FIG. 8, focusing lens 252 focuses the differentwavelength components in a collimated light from VIPA 240 at differentpoints on the mirror 254. More specifically, longer wavelength 264focuses at point 272, center wavelength 266 focuses at point 270 andshorter wavelength 268 focuses at point 274. Then, longer wavelength 264returns to a virtual image 260, which is closer to beam waist 262, ascompared to center wavelength 266. Shorter wavelength 268 returns to avirtual image 260, which is farther from, beam waist 262, as compared tocenter wavelength 266.

[0038] By adjusting the distances between the combination of the VIPA240/lens 252, and the mirror 254, the amount of chromatic dispersionproduced may be adjusted. Thus, at step 660 of FIG. 6, manipulating thelocations of the VIPA 240/lens 252 combination, and the mirror 254performs the adjustment for dispersion compensation.

[0039]FIG. 9 illustrates a second preferred embodiment of a TDC 900,which may be used, with the method of the present invention. This secondembodiment provides discretely tunable dispersion compensation. The TDC900 includes a 1×N optical switch 902. The switch 902 is capable ofrouting the input signal along one of N possible paths. Each path has aconventional fixed dispersion compensator, 904-1 through 904-n, whichprovides different amounts of dispersion. The compensated signal is thenoutput through another 1×N optical switch 906. Thus, the secondembodiment of a TDC 900 is discretely tunable. At step 660 of FIG. 6,the adjustment for dispersion compensation is performed with the opticalswitch 902 routing the input signal to the path which will adjust thechromatic dispersion by a certain amount based upon the command sent bythe EDA 210.

[0040]FIG. 10 illustrates a third embodiment of a TDC 1000, which may beused, with the method of the present invention. This third embodimentcouples a TDC 900 of the second embodiment with a TDC 700 of the firstembodiment. With this TDC 1000, coarse dispersion compensation isprovided by the TDC 900 through the routing of the input signal alongone of N possible paths. The coarsely compensated signal is then finelytuned by the TDC 700.

[0041] One advantage of this embodiment is the speed at which dispersioncompensation occurs. When dispersion compensation occurs with the TDC700 alone, the TDC 700 must provide a large range of dispersion. Butcoupled with the TDC 900, the dispersion adjustment required by the TDC700 is smaller, resulting in a faster compensation speed.

[0042] For example, assume that the optical signal, which enters TDC1000 with a dispersion of −1800 nm, and the desired dispersion, is +650nm. If the TDC 700 was alone, it must compensate for 1450 nm. However,with the TDC 1000, TDC 900 can provide a coarse compensation value of,for example, 1400 nm. Then TDC 700 only need to provide a finecompensation value of 50 nm. A small compensation value may be providedfaster than a larger one. Thus, the TDC 1000 has the added advantage ofa faster compensation speed than the first embodiment.

[0043] Other advantages of the third embodiment include the ability tofine-tune the dispersion compensation and to tune for a wide range ofdispersion. When dispersion compensation occurs with the TDC 900 alone,the compensation value is limited to the discrete values provided by thefixed dispersion compensators 904-1 through 904-n. But coupled to TDC700, the dispersion value is not so limited. The TDC 700 may be used tofine-tune the dispersion. By combining the fixed dispersion compensator900 with the TDC 700, tuning for a wider range of dispersion is possiblethan with either of the compensators alone.

[0044] An important aspect of the present invention is the fact that thesteps illustrated in FIGS. 5 and 6 are performed automatically, i.e.,without the need for a network operator to manually tune the TDC's or tomanually analyze the shape of the optical signal. This saves the networkoperator considerable time and raises the reliability of the networkbecause human error will be minimized.

[0045] The method of tunable dispersion compensation of the presentinvention is not limited to use at particular locations in the network.It may be used to provide dispersion compensation for the network as awhole. FIG. 11 is a flow chart illustrating a preferred embodiment of amethod of tunable dispersion compensation of the optical network inaccordance with the present invention. First, dispersion compensationvalues are provided to a plurality of TDC's in the optical network, viastep 1110. in the preferred embodiment, the values are remotelydownloaded to the discretely switched compensators and/or the TDC's. Thedispersion values may be provided to these TDC's simultaneously,randomly, or based on some other selective method. Next, it isdetermined if the optical signal at each of the TDC's has the desiredshape, via step 1120. If not, then the dispersion of the optical signalat each of the TDC's is automatically adjusted until the desired shapeis obtained, via step 1130.

[0046] The dispersion compensation of the optical network can beimplemented in several ways. FIG. 12 is a flow chart illustrating afirst preferred embodiment of the method of providing tunable dispersioncompensation to an optical network in accordance with the presentinvention. First, dispersion values are provided to all of the TDC's inthe network simultaneously, via step 1210. If the network administratorknows the fiber types in the network and the length of the fiber links,and the dispersion tolerance of the Transmitter/Receiver pair, then theadministrator may calculate approximate dispersion compensation values.These values may then be provided to the corresponding TDC's in thenetwork. Next, it is determined if the optical signal at a location inthe network has the desired shape, via step 1220. The location may beany point of interest in the network. Then, the dispersion of theoptical signal at each TDC is automatically adjusted, via step 1230.Information concerning the adjusted optical signal at the location isfed back to each of the TDC's, via step 1240. The dispersion of theoptical signal is then readjusted based upon the feedback information,via step 1560. This feedback and readjustment continues until theoptical signal at the location has the desired shape.

[0047]FIG. 13 is a flow chart illustrating a second preferred embodimentof the method of providing tunable dispersion compensation in theoptical network in accordance with the present invention. In thisembodiment, dispersion values are provided to the TDC's in the opticalnetwork in a sequential manner, i.e., the TDC's tune the dispersion oneat a time. First, a dispersion compensation value is provided to one ofthe TDC's, via step 1310. Next, it is determined if the optical signalat a location in the optical network has the desired shape, via step1320. The dispersion of the optical signal at the TDC's is thenautomatically adjusted, via step 1330. Information concerning theadjusted optical signal at the location is fed back to the TDC, via step1340. Based upon the feedback information, the dispersion is thenautomatically adjusted, via step 1350. The feedback and readjustmentcontinues until the optical signal has the desired shape. Next, it isdetermined if there is another TDC in the network, which needs to beadjusted, via step 1360. If there is, then steps 1310-1350 are repeateduntil there are no more TDC's in the network, which need to be adjusted.

[0048]FIG. 14 is a flow chart illustrating a third preferred embodimentof the method of providing tunable dispersion compensation in theoptical network in accordance with the present invention. First,dispersion compensation values are provided to less than all of theTDC's in the optical network, via step 1410. The dispersion values maybe provided to these TDC's simultaneously, randomly, or based on someother selective method. Next, it is determined if the optical signal ata location in the network has a desired shape, via step 1420. If not,then the dispersion is automatically adjusted, via step 1430. Feedbackis then provided concerning the optical signal at the TDC at thelocation, via step 1440. Then, the dispersion of the optical signal isautomatically readjusted at the TDC based upon the feedback until theoptical signal at the location has the desired shape, via step 1450. Inthis embodiment, dispersion values are provided first to a group ofTDC's in the network. Then, the TDC's in the network fine-tune theoptical signal.

[0049] Which TDC's are provided the dispersion values in the thirdpreferred embodiment in step 1410 may be decided based on severaldifferent factors. For example, the group could comprise TDC's relatingto a particular fiber type; the group could comprise TDC's with thehighest dispersion compensation values; a statistical averaging of thedispersion compensation values may be provided to the a group of TDC's;or the group could comprise TDC's in a particular geographic location inthe optical network. Other factors may be used without departing fromthe spirit and scope of the present invention.

[0050] A powerful added advantage of the method of the present inventionis the gathering of valuable dispersion information by the EDA 210. Thisinformation can be used in various ways to increase the ease in whichthe optical network may be modified. These ways are disclosed inco-pending U.S. patent application Ser. No. (JAS1018P, filed . Applicanthereby incorporates this patent application by reference.

[0051] One way the information may be used is to approximate the amountof fiber dispersion in the field in each fiber in the system. Forexample, in the system illustrated in FIG. 2, the total dispersion ofthe optical signal as it enters the system at TDC 204 can be determinedthrough the eye optimization function of EDA 210. Also known is thedispersion of the optical signal as it enters TDC 206 and the receiverdispersion tolerance at Transceiver 212. From this information, theapproximate (guessed) amount of dispersion in the fiber 214 may becalculated by subtracting the dispersion at TDC 206 and the transceiver212 from the total dispersion of the signal as it entered the system.The calculation involves taking the total sum of the TDC's for each linkand adding (or subtracting the transceiver tolerance) and the result isthe fiber link dispersion. This calculation may be made for every fiberin a system, and for every system in the network. Thus, in this manner,the amount of fiber link dispersion can be more accurately known foreach fiber in the network. Today, the network operator should rolltraffic off the link and measure each link using a costly fielddispersion measurement system, or the operator measures the link'sdispersion as the cable is spliced or installed. This is costly and alsoresults in potential lost revenue while the traffic is being rolled, aswell as vast human labor and field coordination activities.

[0052] The amount of dispersion for each fiber may then be stored in adatabase and used to create a network dispersion map. A dispersion mapallows for the upgrading or repairing of various systems components withmore ease than under conventional dispersion compensation methods.Without this dispersion map, each time an upgrade or repair of a fiberin the system is desired, a network operator must manually measure thedispersion of the system in order to calculate the approximate length offiber needed for the upgrade or repair so that dispersion in the systemis properly compensated. Once the new system is installed, thedispersion compensators are manually adjusted to accommodate the actualdispersion needed by the new system. This will become extremelyimportant for short pulsed systems such as 40 Gb/s. The amount of humanlabor required is burdensome.

[0053] But, with a dispersion map created with the method of the presentinvention, the approximate dispersion of each fiber of the system isalready known. The approximate length of the new fiber necessary forincorporation into the system may be readily calculated. Once the newfiber is installed, the optical signal will be automatically compensatedfor dispersion by the TDCs to obtain the desired signal shape. Thus, theamount of human labor required is considerably less than underconventional methods. The system may be upgraded or repaired in ashorter amount of time. Also, the operator has virtually no inventorysince only a few TDCs are needed for the entire fiber plant.

[0054] The dispersion map may be used to provide different dispersionsignatures, which may be used to perform other modifications to theoptical network. Conventionally, dispersion signatures are obtainedutilizing optical fiber modeling software, and the model is then testedin a laboratory. However, with the dispersion signature obtained fromthe dispersion map of the present invention, the dispersion signaturecan be modeled based upon real-life values, can be easily downloaded tothe TDC's, and dispersion values may be fed back to the TDC's ifnecessary. The dispersion signature thus reduces the amount of humanlabor required to modify the optical network.

[0055] Another added advantage of the method in accordance with thepresent invention is the ability to add or “dial in” dispersion to afiber to change its characteristics. This may be desirable whenadditional dispersion is required to make the fiber compatible with theremainder of the system. For example, assume a system uses theDispersion Managed Soliton pulse format for its signal, which is knownin the art. This pulse format is designed for use with conventionalnon-dispersion shifted fibers which have steep dispersion slopes. Othercomponents in the system are thus installed to function with fibers withthese characteristics. But, assume that the network operators decide tomove the conventional system to a new site where the fibers in thesystem are Lucent True Wave fibers instead of the conventionalnon-dispersion shifted fiber. True wave fibers have shallow dispersionslopes in relation to non-dispersion shifted fibers. However, the oldersystem would not be compatible with the true wave fibers because theywere designed to function with fibers having non-dispersion shiftedfiber characteristics. With the method of the present invention, thenetwork operator may change the characteristics of the true wave fibersby dialing in large, additional dispersion so that they emulatenon-dispersion-shifted fibers. In this manner, the true wave fibers arenow approximately compatible with the remaining components in thesystem, or accommodate new pulse formats such as Dispersion ManagedSolitons.

[0056] The ability to dial in dispersion also provide the networkoperator with an advantage when switching wavelength channels from onefiber in a link or system to another fiber in another cable link orsystem. In the future, with optical switching the network operator willbe able to switch wavelengths from one fiber to another. The TDC conceptwill allow the operator to future-proof their network. Sometimes, theother fiber routes or cables support a pulse format or bit ratesdifferent than the format of the operating signal. The method of thepresent invention allows the network operator to pre-tune the network inanticipation of the differences between the signal's format or bit rate,and the other fiber before the switch is actually performed. Thisaffords the operator with quicker maintenance switching, restorationspeed, and provisioning times.

[0057] A method and system for compensating for chromatic dispersion inan optical network has been disclosed. The method and system of thepresent invention automatically compensates for dispersion in an opticalsystem. This saves the network operator considerable time and raises thereliability of the network because human error will be minimized. Themethod and system could be used to provide a continuous range ofdispersion compensation to particular locations in the network or to thenetwork as a whole. It has the added advantage of providing networkoperators with information, which may be stored in a database and usedto produce dispersion maps for the network. Dispersion maps help networkdesigners understand and remotely control fiber characteristics whencomponent upgrades or repairs are desired. The dispersion maps providedispersion signatures useful in performing modifications of the network.It also has the added advantage of providing network operators with theability to dial in dispersion which is useful for changing thecharacteristics of the fibers in the system and for pre-tuning a systemin a switching of wavelengths from one system to another. Finally, itaffords the operator the ability to “guess” at the fiber plantdispersion without consuming valuable time and energy measuring everyfiber link.

[0058] Although the present invention has been described in accordancewith the embodiments shown, one of ordinary skill in the art willreadily recognize that there could be variations to the embodiments andthose variations would be within the spirit and scope of the presentinvention. Accordingly, many modifications may be made by one ofordinary skill in the art without departing from the spirit and scope ofthe appended claims.

What is claimed is:
 1. A method for compensating for chromaticdispersion in an optical network, comprising the steps of: (a) analyzingan optical signal at a first location in the optical network; (b)determining if the optical signal has a desired shape at the firstlocation; (c) automatically adjusting a dispersion of the optical signalat a second location in the optical network if the optical signal doesnot have the desired shape; (d) providing feedback concerning the shapeof the adjusted optical signal at the first location; and (e)automatically readjusting the dispersion of the optical signal at thesecond location based upon the feedback until the optical signal at thefirst location has the desired shape.
 2. The method of claim 1 , whereinthe analyzing step (a) comprises: (a1) obtaining parameters of theoptical network; (a2) calculating an initial set point for adjustment ofthe optical signal; (a3) sending the adjustment command to a tunabledispersion compensator at the second location; and (a4) analyzing theshape of the optical signal at the first location.
 3. The method ofclaim 2 , wherein the obtaining step (a1) comprises: (a1i) obtaining afiber type of the optical network; (a1ii) obtaining a distance betweenthe first and second locations; and (a1iii) obtaining a dispersiontolerance at the first location.
 4. The method of claim 1 , wherein theshape of the optical signal is measured using an eye area method, anoptical signal to noise method, a dBQ method, or a bit error ratemethod.
 5. The method of claim 1 , wherein the automatically adjustingstep (c) and the automatically readjusting step (e) is performed by atunable dispersion compensator, the tunable dispersion compensatorcomprising: a virtually imaged phased array (VIPA); a lens opticallycoupled to the VIPA; and a mirror optically coupled to the lens, whereinthe dispersion compensation may be tuned by adjusting the distancesbetween the VIPA lens, and mirror combination.
 6. The method of claim 1, wherein the automatically adjusting step (c) and the automaticallyreadjusting step (e) is performed by a tunable dispersion compensator,the tunable dispersion compensator comprising: a first tunabledispersion compensator capable of providing a fixed amount of dispersioncompensation; and a second tunable dispersion compensator, coupled tothe first tunable dispersion compensator, capable of providing acontinuous range of dispersion compensation.
 7. The method of claim 6 ,wherein the first tunable dispersion compensator comprises: a one-by-nswitch, n being an integer greater than one; n dispersion compensators,each capable of providing a fixed amount of dispersion compensation,wherein the one-by-n switch is capable of routing the optical signal toany one of the n dispersion compensators; and another one-by-n switchoptically coupled to the dispersion compensator to which the opticalsignal is routed.
 8. The method of claim 6 , wherein the second tunabledispersion compensator comprises: a virtually imaged phased array(VIPA); a lens optically coupled to the VIPA; and a mirror opticallycoupled to the lens, wherein the dispersion compensation may be tuned byadjusting the distances between the VIPA lens, and mirror combination.9. The method of claim 1 , wherein the first location is at a receiver.10. The method of claim 1 , wherein the second location is at a tunabledispersion compensator.
 11. A system for compensating for chromaticdispersion in an optical network, comprising: an optical signal analyzerat a first location in the optical network for determining a shape ofthe optical signal at the first location; a tunable dispersioncompensator capable of providing a continuous range of dispersioncompensation at a second location in the optical network; and a linkcoupled to the tunable dispersion compensator and the optical signalanalyzer for providing feedback from the optical signal analyzer to thetunable dispersion compensator concerning the shape of the opticalsignal at the first location, wherein a dispersion of the optical signalis adjusted by the tunable dispersion compensator based upon thefeedback until the optical signal has a desired shape.
 12. The system ofclaim 11 , wherein the optical signal analyzer comprises: means forobtaining parameters of the optical network; means for calculating aninitial set point for adjustment of the optical signal; means forsending the adjustment command to the tunable dispersion compensator atthe second location; and means for analyzing the shape of the opticalsignal at the first location.
 13. The system of claim 12 , wherein theparameters comprise: a fiber type of the optical network; a distancebetween the first and second locations; and a dispersion tolerance atthe first location.
 14. The system of claim 11 , wherein the shape ofthe optical signal is measured using an eye area method, an opticalsignal to noise method, a dBQ method, or a bit error rate method. 15.The system of claim 11 , wherein the tunable dispersion compensatorcomprises: a virtually imaged phased array (VIPA); a lens opticallycoupled to the VIPA; and a mirror optically coupled to the lens, whereinthe dispersion compensation may be tuned by adjusting the distancesbetween the VIPA lens, and mirror combination.
 16. The system of claim11 , wherein the tunable dispersion compensator comprises: a firsttunable dispersion compensator capable of providing a fixed amount ofdispersion compensation; and a second tunable dispersion compensator,coupled to the first tunable dispersion compensator, capable ofproviding a continuous range of dispersion compensation.
 17. The systemof claim 16 , wherein the first tunable dispersion compensatorcomprises: a one-by-n switch, n being an integer greater than one; ndispersion compensators, each capable of providing a fixed amount ofdispersion compensation, wherein the one-by-n switch is capable ofrouting the optical signal to any one of the n dispersion compensators;and another one-by-n switch optically coupled to the dispersioncompensator to which the optical signal is routed.
 18. The system ofclaim 16 , wherein the second tunable dispersion compensator comprises:a virtually imaged phased array (VIPA); a lens optically coupled to theVIPA; and a mirror optically coupled to the lens, wherein the dispersioncompensation may be tuned by adjusting the distances between the VIPAlens, and mirror combination.
 19. The system of claim 11 , wherein thefirst location is at a receiver.
 20. The system of claim 11 , whereinthe second location is at a tunable dispersion compensator.
 21. A methodfor compensating for chromatic dispersion in an optical network,comprising the steps of: (a) providing dispersion compensation values toa plurality of tunable dispersion compensators in the optical network;(b) determining if an optical signal at a location in the opticalnetwork has a desired shape; (c) automatically adjusting a dispersion ofthe optical signal at each of the tunable dispersion compensators; and(d) automatically readjusting the dispersion of the optical signal basedupon a feedback concerning the adjusted optical signal, until theoptical signal at the location has the desired shape.
 22. The method ofclaim 21 , wherein the providing step (a) comprises: (a1) providing adispersion value to a tunable dispersion compensator of the plurality oftunable dispersion compensators.
 23. The method of claim 21 , whereinthe optical signal is measured using an eye area method, an opticalsignal to noise method, a dBQ method, or a bit error rate method. 24.The method of claim 21 , wherein the determining step (b) comprises:(b1) determining if a shape of the optical signal at the location hasthe desired shape.
 25. The method of claim 21 , wherein theautomatically readjusting step (d) comprises: (d1) automaticallyadjusting a dispersion of the optical signal at the tunable dispersioncompensators based upon the feedback until the optical signal at thelocation has a desired shape, if the optical signal does not have thedesired shape; (d2) determining if another tunable dispersioncompensator in the plurality of tunable dispersion compensators requiresadjustment; and (d3) repeating steps (a)-(d2) if another tunabledispersion compensator-requires adjustment.
 26. The method of claim 21 ,wherein the providing step (a) comprises: (a1) providing dispersioncompensation values to less than all of the plurality of tunabledispersion compensators in the optical network.
 27. The method of claim26 , wherein the dispersion compensation values are provided to the lessthan all of the plurality of tunable dispersion compensatorssimultaneously.
 28. The method of claim 26 , wherein the dispersioncompensator values are provided to the less than all of the plurality oftunable dispersion compensators based upon a predetermined factor. 29.The method of claim 28 , wherein the predetermined factor comprisesfiber types in the optical network.
 30. The method of claim 28 , whereinthe predetermined factor comprises a predetermined level of dispersioncompensation.
 31. The method of claim 28 , wherein the predeterminedfactor comprises a statistical averaging of the dispersion compensatorvalues.
 32. The method of claim 28 , wherein the predetermined factorcomprises a geographic location in the optical network.
 33. A method forcompensating for chromatic dispersion in an optical network, comprisingthe steps of: (a) analyzing an optical signal at a first location in theoptical network; (b) determining if the optical signal has a desiredshape at the first location; (c) automatically adjusting a dispersion ofthe optical signal utilizing a tunable dispersion compensator at asecond location in the optical network if the optical signal does nothave the desired shape; (d) providing feedback concerning the shape ofthe adjusted optical signal at the first location; and (e) automaticallyreadjusting the dispersion of the optical signal based upon the feedbackuntil the desired shape at the first location is obtained.
 34. Themethod of claim 33 , wherein the analyzing step (a) comprises: (a1)obtaining parameters of the optical network; (a2) calculating an initialset point for adjustment of the optical signal; (a3) sending theadjustment command to the tunable dispersion compensator; and (a4)analyzing the shape of the optical signal at the first location.
 35. Themethod of claim 34 , wherein the obtaining step (a1) comprises: (a1i)obtaining a fiber type of the optical network; (a1ii) obtaining adistance between the first location and the tunable dispersioncompensator; and (a1iii) obtaining a dispersion tolerance at the firstlocation.
 36. The method of claim 33 , wherein the shape of the opticalsignal is measured using an eye area method, an optical signal to noisemethod, a dBQ method, or a bit error rate method.
 37. The method ofclaim 33 , wherein the tunable dispersion compensator comprises: avirtually imaged phased array (VIPA); a lens optically coupled to theVIPA; and a mirror optically coupled to the lens, wherein the dispersioncompensation may be tuned by adjusting the distances between the VIPAlens, and mirror combination.
 38. The method of claim 33 , wherein thetunable dispersion compensator comprises: a first tunable dispersioncompensator capable of providing a fixed amount of dispersioncompensation; and a second tunable dispersion compensator, coupled tothe first tunable dispersion compensator, capable of providing acontinuous range of dispersion compensation.
 39. The method of claim 38, wherein the first tunable dispersion compensator comprises: a one-by-nswitch, n being an integer greater than one; n dispersion compensators,each capable of providing a fixed amount of dispersion compensation,wherein the one-by-n switch is capable of routing the optical signal toany one of the n dispersion compensators; and another one-by-n switchoptically coupled to the dispersion compensator to which the opticalsignal is routed.
 40. The method of claim 38 , wherein the secondtunable dispersion compensator comprises: a virtually imaged phasedarray (VIPA); a lens optically coupled to the VIPA; and a mirroroptically coupled to the lens, wherein the dispersion compensation maybe tuned by adjusting the distances between the VIPA lens, and mirrorcombination.
 41. A system for compensating for chromatic dispersion inan optical network, comprising: means for analyzing an optical signal ata first location in the optical network; means for determining if theoptical signal has a desired shape at the first location; means forautomatically adjusting a dispersion of the optical signal utilizing atunable dispersion compensator at a second location in the opticalnetwork if the optical signal does not have the desired shape; and meansfor providing feedback concerning the shape of the adjusted opticalsignal at the first location, wherein the dispersion of the opticalsignal may be readjusted based upon the feedback until the opticalsignal at the first location has the desired shape.
 42. The system ofclaim 41 , wherein the analyzing means comprises: means for obtainingparameters of the optical network; means for calculating an initial setpoint for adjustment of the optical signal; means for sending theadjustment command to the tunable dispersion compensator; and means foranalyzing the shape of the optical signal at the first location.
 43. Thesystem of claim 42 , wherein the obtaining means comprises: means forobtaining a fiber type of the optical network; means for obtaining adistance between the first location and the tunable dispersioncompensator; and means for obtaining a dispersion tolerance at the firstlocation.
 44. The system of claim 41 , wherein the determining meanscomprises an eye area measurement, an optical signal to noisemeasurement, a dBQ measurement, or a bit error rate measurement.
 45. Thesystem of claim 41 , wherein the tunable dispersion compensatorcomprises: a virtually imaged phased array (VIPA); a lens opticallycoupled to the VIPA; and a mirror optically coupled to the lens, whereinthe dispersion compensation may be tuned by adjusting the distancesbetween the VIPA lens, and mirror combination.
 46. The system of claim41 , wherein the tunable dispersion compensator comprises: a firsttunable dispersion compensator capable of providing a fixed amount ofdispersion compensation; and a second tunable dispersion compensator,coupled to the first tunable dispersion comparator capable of providinga continuous range of dispersion compensation.
 47. The system of claim46 , wherein the first tunable dispersion compensator comprises: aone-by-n switch, n being an integer greater than one; n dispersioncompensators, each capable of providing a fixed amount of dispersioncompensation, wherein the one-by-n switch is capable of routing theoptical signal to any one of the n dispersion compensators; and anotherone-by-n switch optically coupled to the dispersion compensator to whichthe optical signal is routed.
 48. The method of claim 46 , wherein thesecond tunable dispersion compensator comprises: a virtually imagedphased array (VIPA); a lens optically coupled to the VIPA; and a mirroroptically coupled to the lens, wherein the dispersion compensation maybe tuned by adjusting the distances between the VIPA lens, and mirrorcombination.
 49. A method for compensating for chromatic dispersion inan optical network, comprising the steps of: (a) obtaining parameters ofthe optical network; (b) calculating an initial set point for adjustmentof an optical signal; (c) sending the adjustment command to a tunabledispersion compensator at a first location, the tunable dispersioncompensator capable of providing a continuous range of dispersioncompensation; (d) analyzing a shape of the optical signal at a secondlocation; (e) determining if the optical signal has a desired shape atthe second location; (f) automatically adjusting a dispersion of theoptical signal at the first location if the optical signal does not havethe desired shape; (g) providing feedback concerning a shape of theadjusted optical signal at the second location; and (h) automaticallyreadjusting the dispersion compensation of the optical signal at thefirst location based upon the feedback until the optical signal at thesecond location has the desired shape.
 50. The method of claim 49 ,wherein the obtaining step (a) comprises: (a1) obtaining a fiber type ofthe optical network; (a2) obtaining a distance between the secondlocation and the tunable dispersion compensator; and (a3) obtaining adispersion tolerance at the second location.
 51. The method of claim 49, wherein the shape of the optical signal is measured using an eye areamethod, an optical signal to noise method, a dBQ method, or a bit errorrate method.
 52. The method of claim 49 , wherein the tunable dispersioncompensator comprises: a virtually imaged phased array (VIPA); a lensoptically coupled to the VIPA; and a mirror optically coupled to thelens, wherein the dispersion compensation may be tuned by adjusting thedistances between the VIPA lens, and mirror combination.
 53. The methodof claim 49 , wherein the tunable dispersion compensator comprises: afirst tunable dispersion compensator capable of providing a fixed amountof dispersion compensation; and a second tunable dispersion compensator,coupled to the first tunable dispersion compensator, capable ofproviding a continuous range of dispersion compensation.
 54. The methodof claim 53 , wherein the first tunable dispersion compensatorcomprises: a one-by-n switch, n being an integer greater than one; ndispersion compensators, each capable of providing a fixed amount ofdispersion compensation, wherein the one-by-n switch is capable ofrouting the optical signal to any one of the n dispersion compensators;and another one-by-n switch optically coupled to the dispersioncompensator to which the optical signal is routed.
 55. The method ofclaim 53 , wherein the second tunable dispersion compensator comprises:a virtually imaged phased array (VIPA); a lens optically coupled to theVIPA; and a mirror optically coupled to the lens, wherein the dispersioncompensation may be tuned by adjusting the distances between the VIPAlens, and mirror combination.
 56. A method for compensating forchromatic dispersion in an optical network, comprising the steps of: (a)providing dispersion compensation values to a plurality of tunabledispersion compensators in the optical network simultaneously; (b)determining if an optical signal at a location in the optical networkhas a desired shape; and (c) automatically adjusting a dispersion of theoptical signal at each of the tunable dispersion compensators; (d)providing feedback concerning the adjusted optical signal at thelocation to each of the tunable dispersion compensators; and (e)automatically readjusting the dispersion of the optical signal at eachof the tunable dispersion compensators based upon the feedback until theoptical signal at the location has the desired shape.
 57. A method forcompensating for chromatic dispersion in an optical network, comprisingthe steps of: (a) providing a dispersion value to a tunable dispersioncompensator of a plurality of tunable dispersion compensators in theoptical network; (b) determining if the optical signal at a location inthe optical network has a desired shape; (c) automatically adjusting adispersion of the optical signal at the tunable dispersion compensator;(d) providing feedback concerning the adjusted optical signal at thelocation to the tunable dispersion compensator; (e) automaticallyreadjusting the dispersion of the optical signal at the tunabledispersion compensator based upon the feedback until the optical signalat the location has the desired shape; (f) determining if anothertunable dispersion compensator in the plurality of tunable dispersioncompensators requires adjustment; and (g) repeating steps (a)-(f) ifanother tunable dispersion compensator requires adjustment.
 58. A methodfor compensating for chromatic dispersion in an optical network,comprising the steps of: (a) providing dispersion compensation values toless than all of a plurality of tunable dispersion compensators in theoptical network; (b) determining if an optical signal at a location inthe optical network has a desired shape; (c) automatically adjusting adispersion of the optical signal at each of the tunable dispersioncompensators; (d) providing feedback concerning the adjusted opticalsignal at the location to the tunable dispersion compensator; and (e)automatically readjusting the dispersion of the optical signal at thetunable dispersion compensator based upon the feedback until the opticalsignal at the location has the desired shape.
 59. The method of claim 58, wherein the dispersion compensation values are provided to the lessthan all of the plurality of tunable dispersion compensatorssimultaneously.
 60. The method of claim 58 , wherein the dispersioncompensator values are provided to the less than all of the plurality oftunable dispersion compensators based upon a predetermined factor. 61.The method of claim 60 , wherein the predetermined factor comprisesfiber types in the optical network.
 62. The method of claim 60 , whereinthe predetermined factor comprises a predetermined level of dispersioncompensation.
 63. The method of claim 60 , wherein the predeterminedfactor comprises a statistical averaging of the dispersion compensatorvalues.
 64. The method of claim 60 , wherein the predetermined factorcomprises a geographic location in the optical network.